The present invention relates to an electronic circuit of the type in which a load circuit is driven by a battery and a voltage drop circuit provides a dropped voltage.
In an electronic circuit using a battery as a drive source, such as an electronic timepiece, a liquid crystal display device with small power consumption for reducing power consumption has been normally used. Further, to reduce the number of the output terminals as much as possible for making the circuit well adaptable for LSI fabrication, a multiplex display system is frequently employed. At least three voltages are required for the multiplex display system. For this reason, a conventional electronic timepiece using a battery for the drive source is provided with a voltage drop circuit or a voltage raising circuit.
In a conventional electronic timepiece with a voltage drop circuit 2, as shown in FIG. 1, the voltage drop circuit 2 produces an output voltage V.sub.SS /2 at the output terminal 8 by halving the voltage V.sub.SS of the battery 1. The voltage drop circuit 2 is generally called "voltage halves". With the circuit arrangement, three voltages are supplied: V.sub.SS /2 from the voltage drop circuit 2, a reference potential, V.sub.DD as a ground potential, and the battery voltage V.sub.SS.
As shown in FIG. 1, the voltage drop circuit 2 includes select switches 3 and 4 each having a movable contact a and stationary contacts b and c, a switch 5 having a movable contact a and a stationary contact b, and capacitors 6 and 7 with equal capacitances. The positive electrode of the battery 1 is coupled to the ground potential V.sub.DD and its negative electrode at the potential V.sub.SS is connected to the stationary contact b of the select switch 3. The movable contact a of the switch 3 is connected to one end of the capacitor 6 of which the other end is connected to the movable contact a of the switch 4. The stationary contact c of the switch 3 and the stationary contact b of the switch 4 are connected to the output terminal 8. The capacitor 7 is connected between the output terminal 8 and the ground. The stationary contact c of the switch 4 is connected to ground potential point. The movable contact a of the switch 5 is connected to the negative electrode of the battery 1, and its stationary contact b is connected to the output terminal 8.
The electronic timepiece is provided with a load circuit, for example, a buzzer 9 for alarm. The buzzer 9 is connected at one end to the negative electrode of the battery, through an NPN switch transistor 10 serving as a power source switch. When the transistor 10 is ON, electrical power is fed from the battery 1 to the buzzer 9. The other end of the buzzer 9 is connected to the ground potential.
A constant frequency divider 11 in FIG. 1 is connected to the select switches 3 and 4 and the switch transistor 10, and produces switch control signals for controlling the switch operations of these switches 3, 4, and 10. A control signal generator 12 is connected to the switch 5 and produces a switch control signal for controlling the operation of the switch 5.
With such a construction, the switch 5 responds to a control signal generated from the control signal generator 12 (through an operator's external operation) for turning on the switch 5 which is turned on only when the voltage drop circuit 2 starts to operate. The switches 3 and 4 are repeatedly switched in a synchronous manner by the switch control signal at a frequency of 256 Hz generated from the constant frequency divider 11. When the stationary contact a of the switch 3 is set to the stationary contact b, the movable contact a of the switch 4 is also set to the stationary contact b. Similarly, when the movable contact a of the switch 3 is turned to the stationary contact c, the movable contact a of the switch 4 is turned to the stationary contact c.
When the movable contacts a of the switches 3 and 4 are connected to the stationary contacts b, the two capacitors 6 and 7 are connected in series across the battery 1. Since the capacitances of the capacitors 6 and 7 are normally selected to be equal to each other, these capacitors are charged up to 1/2 voltage of the battery voltage V.sub.SS, so long as the voltage V.sub.SS of the battery 1 is kept constant. Accordingly, the voltage V.sub.SS /2 is derived from the output terminal 8.
When the switches 3 and 4 are turned from the movable contacts a to the contacts c, the two capacitors 6 and 7 are separated from the battery 1, so that they are connected in parallel between the output terminal 8 and ground. In this switching state of these switches, the charges stored in capacitors 6 and 7 when switches 3 and 4 were in the previous switching state are taken out from the output terminal 8 in the form of an output voltage.
The switching operation of the switches 3 and 4 is repeated at a high speed determined by 256 Hz frequency off the control signal. Therefore, the amount of charge discharged is very small and the potential at the output terminal 8 little changes. Thus, the voltage V.sub.SS /2 at the output terminal 8 is almost constant, so long as the capacitors 6 and 7 are charged at the voltage V.sub.SS /2. In this way, the prior electronic circuit can provide three voltages the: V.sub.SS /2, V.sub.SS, and V.sub.DD.
For sounding the buzzer 9, responding to an external operation by an operator, the constant frequency divider 11 generates a drive control signal of 2,048 Hz frequency which in turn is applied to the base of the switch transistor 10. The transistor 10 is driven at 2,048 Hz frequency of the drive control signal to allow the voltage V.sub.SS to be applied to the buzzer 9. The result is the sounding of the buzzer 9 at the 2,048 Hz frequency of the drive control signal from the constant frequency divider 11.
In general, the frequency within the range from 256 Hz to 1,024 Hz is used for the switch control signal for switching the switches 3 to 5, while the frequency of the drive control signal for driving the transistor 10 and the buzzer 9 is 2,048 Hz to 4,096 Hz. Thus, both the signals are different in the frequencies and further not timed to each other. Therefore, when the battery is exhausted causing it to have a large internal resistance, if the buzzer 9 as the load circuit is operated, the output voltage V.sub.SS greatly changes, as shown in FIG. 2A.
With the great variation of the voltage V.sub.SS, the amount of the charge stored in the capacitors 6 and 7 is also reduced. Therefore, when the capacitors 6 and 7 are separated from the battery 1, a voltage lower than the V.sub.SS /2 is produced at the output terminal 8. Such an unstable output voltage brings about an abnormal display operation of the timepiece. For this reason, when the prior electronic circuit of FIG. 1 is used for a timepiece, the battery, even if it has an effective amount of the battery capacity, must be discarded and replaced by a fresh one.